Storage tube target structure having attached shield ring



Mw HW G. D. CARTWRIGHT ETAL. wm

STORAGE TUBE TARGET STRUCTURE HAVING ATTACHED SHIELD RING Filed Nov. l2,1964 United States Patent "i 3,319,1.03 STRAGE TUBE TARGET STRUCTUREHAVTNG ATTACHED SHHELD RTN@ Gerald D. Cartwright, Lititz, and )Edgar M.Smith, Miliersviille, Pa., assignors to Radio Corporation of America, acorporation of Delaware Filed Nov. l2, 1964, Ser. No. 416,433 2 Claims.(Cl. 313-68) This invention relates to display storage tubes andparticularly to an improved target for such tubes.

One type of display storage tube comprises an envelope having a glassfa-ceplate coated on its inner surface with a phosphor screen that emitslight when subjected to electron impingement. In the operation of thistype of storage tube, useful light emission fr-om the screen occurs`only `during a Viewing cycle.

To provide such useful light emission from the screen, the tube includesa perforated target closely spaced from the screen. An electrostaticcharge pattern produced on the target modulates a relatively lowvelocity electron beam which iioods the screen during the viewing cycleto provide -a viewable display on the screen corresponding to theelectrostatic charge pattern.

The charge pattern on the target is produced during a writing -cycle ofoperation and is effected by scanning a relatively high velocityelectron beam across the target. The high velocity electron beam ismodulated by signals derived from a lig-ht pattern to be viewed, to varythe velocity of the beam in response to the modulating signals. Suchvariations in the velocity of the beam, charge an insulating coating onthe target by the phenomenon of secondary electron emission to producethe charge pattern thereon.

The charge pattern so produced on the target is not destroyed by therelatively low velocity viewing beam during the viewing cycle.Consequently, the target, when once charged, is effective to produce adisplay throughout a relatively long period. However, the charge patternon the target can be destroyed fully during a later erase cycle.

Erasure of the electrostatic charge pattern on the target isaccomplished by the viewing beam in combination with the appli-cation ofa positive erase pulse to a conducting mesh substrate on which theinsulating coating of the target is deposited. When so pulsed, theviewing beam becomes an erase beam. Such a positive pulse causeselectrons of the beam to be attracted to the target and to land thereon.A negative charge is thereby produced on the target which effectivelycuts off the viewing beam, and thereby erases the display image.

Several serious problems are involved in a display storage tube of thetype described. One problem is related to the modes of operation of thetube and other problems arise as a consequence of unavoidable errors inapplying an insulating charge-holding coating to the target of the tube.

The problem related to the modes of operation of tube arises as aconsequence of the fact that the erase beam does not discharge allportions of the target in an erasing operation. The portions not erasedare located at the periphery of the target and produce a peripheral orannular ring of intense illumination upon the phosphor screen during asubsequent viewing operation. Presence of the bright ring isobjectionable for several reasons. Light from the ring may be reflectedinternally from glassair surfaces, and thus make information at lowlight levels difficult to see. The ring also makes it `difficult for theobserver to become dark adapted. Furthermore, under conditions of lowambient illumination, the bright ring adds unnecessary light, thusmaking the observation of other close-by instruments diicult.

3 ,3 l 9, l @E Patented May 9, 1967 ICC The bright ring is caused bycontraction vof the viewing beam diameter by the erase pulse so that thebeam diameter is appreciably smaller than the diameter of the target.During an erasing operation, therefore, the erase beam covers a smallerarea of the target than is exposed to the viewing beam in a viewingoperation and the writing beam in a writing operation. This leaves aperipheral region of the target in which charges produced during thewriting operation are incapable of erasure by the erasing beam. After anumber of writing, viewing and erasing cycles, the charge built up inthe peripheral region of the target becomes large enough to produce thebright ring in a subsequent viewing operation.

One prior attempt to solve this problem has involved the application ofan appropriate pulse to an electrode of the tube during the erasingoperation for the purpose of widening the erasing beam. However, thissolution to the problem is not entirely satisfactory because of theexpense involved in providing the components required for the beamwidening pulse, and because the components, as well as the electricalcircuit required for their use, may be subject to malfunctions.

With respect to the several problems presented by unavoidable errors inapplying the charge-holding insulating material to the target, oneproblem concerns a buildup of electrical charges on an annular portionof the target that overlaps an imperforate target support flange. Thisannular portion is so close to the area of the target to be coated thatfrequently during a target coating operation, some of the coatingmaterial is also unavoidably deposited on the annular portion. Thisannular portion is exposed to the relatively wide area scan of thewriting beam and consequently acquires an electrostatic charge from thewriting beam. The annular portion referred to is outside of the targetarea ooded by the viewing and erasing beams, and, therefore, theelectrical charge thereon is built up during each writing operation.This charge nally becomes sufficiently large to produce a voltagebreak-down between the target and phosphor screen. By voltage break-downis meant a voltage difference between the target and the screen of suchmagnitude as to cause electron ow from the target to the adjacentscreen. Since the target is apertured, such electron flow is free tooccur from the charged imperforate annular portion, to the screenthrough apertures in the target. Such voltage break-down or ashoverproduces a bright annular light response on the phosphor screen. Whenthis voltage break-down occurs during a viewing operation, it seriouslyaffects the fidelity of the display presented on the screen.

Another problem occurring because of unavoidable error in coating thetarget is due to failure to coat the target completely. Such failure tocoat the target cornpletely may arise from excessive care in attemptingto avoid coating the annular region of overlap of the target and itssupport ring. In this situation, a portion of the target mesh adjacentto the support ring may remain free of target coating. Such uncoatedportion is objectionable for proper tube operation.

Accordingly, it is an object of the invention to provide sa structurethat simply and economically avoids the problems discussed in theforegoing.

lt is a further object to provide adjacent to the target of the displaystorage tube, conductive means that avoids the problems of (l) brightring on the phosphor screen, (2) voltage break-down between the targetand screen, (3) undesired excitation of the screen through uncoatedportions of the target.

The invention accomplishes the foregoing objects by mounting on thetarget, a ring made of electrically conducting material for coveringcertain regions of the target structure. The regions covered are thosethat give rise to the problems mentioned, and which are on the side ofthe target facing the direction from which the writing, viewing anderasing beams originate. In one example, the conducting material is inthe form of a metal ring. The ring is so shaped and positioned on thetarget that it intercepts electron beams at peripheral portions of thetarget and effectively dissipates the resultant electrical charges atsuch portions through a suitable lead. In this way, operation of thetube is free from the several effects of a bright ring, voltagebreak-down between the target and screen and undesired excitations ofthe screen.

The invention will be described in greater detail by reference to thefolowin'g description taken in connection with the accompanying drawingin which:

FIG. 1 is a schematic View of a display storage tube in which thepresent invention may be used; and

FIG. 2 is a fragmentary sectional view of the tube depicted in FIG. 1and shows the novel target including the conducting ring of theinvention.

The display storage tube shown in FIG. 1 includes an evacuated glassenvelope 10, having a glass faceplate 11. On the inner surface of thefaceplate 11 is a phosphor viewing screen 12. The phosphor viewingscreen 12 is bonded to a transparent conducting film 14 made of metal ormetallic compound such as tin oxide, for example, deposited on the innersurface of the glass faceplate 11. The conducting film 14 is normallymaintained at from about 5,000 to 10,000 volts positive with respect toground. Located next to the phosphor screen 12 is a storage targetassembly \15 consisting of a storage grid 16 and a conducting mesh 18.The storage grid .16 cornprises a relatively thin deposit on the mesh 18of an insulating material such as magnesium tiuoiide having a thicknessof the order of several microns. The insulating material is sufficientlythin so that the size of the openings in the conducting mesh 18 remainsubstantially unchanged. The storage grid or screen 16 is in effect amultiplicity of `storage elements capacitively coupled to the conductingmesh 18.

A collector electrode 20 is located next to the target 15 and spacedabout one-eighth inch therefrom. rI'he collector electrode 20 includes ametallic cylinder 22 supporting a grid structure 24, for-med of a finemetal mesh. The ne mesh collector grid 24, the storage target assembly15 `and viewing screen 12 are closely spaced and in parallel planes.

The viewing section of the storage tube includes a cathode shownschematically at 26, a pair of grids 2S and 30, a conducting coating 32on the inner part of the envelope 10, and a hollow metallic cylinder 34.The hollow metallic cylinder or electrode 34 serves to collimate theelectrons emitted by the cathode 26. When impressed with a positivepulse, this section becomes the erasing section.

The writing sction of the tube includes a cathode 36, four grids 38, 40,42, 44 and electrostatic deflection plates 46 and 48.

The functions of the Writing, viewing and erasing sections are asfollows. The writing section provides a highly focused beam of highvelocity electrons which are scanned across a relatively large areaincluding the storage Agrid 16 and an annularimperforate region 49 (FTG.2) where the storage grid overlaps a target supporting flange 50, forproducing an electrostatic charge pattern thereon. The viewing section,on the other hand, produces a flood beam of electrons of relatively lowvelocity directed to an area smaller than that affected by the writingbeam. During a viewing operation, the electrons of the flood beam eitherpass through the storage grid and land on the phosphor screen 12, or arerepelled by the storage grid to the collector 20.

The flood beam of electrons of the viewing section is also utilized inerasing the display produced by the writing section. The erasingoperation takes place after a viewing operation has been completed.During erasure,

the viewing beam is changed to an erasing beam by :applying a largerpositive potential to the mesh support 18 of the storage grid 16, tocause electrons of the beam to be attracted to, and land on, an area ofthe storage grid. This area is smaller than that affected by the viewingbeam because of the reduced size of the erase beam occasioned by theincreased positive potential applied to the mesh 18. The landingelectrons do not have sufficient energy to produce a secondary emissionin excess of unity. A negative charge consequently is produced on thestorage grid 16, which effectively cuts off the viewing beam and erasesthe displayed image.

In one example of operation of a display storage tube of the typedescribed, the following exemplary potentials may be applied to theseveral electrodes thereof:

Volts Conducting film 14 +5,00() to +10,000 Conducting mesh 18 0 to +5Collector 20 +150 to +210 Collimater 34 +32 to +150 Electrode 32 +10 to+50 Cathode 26 0 Cathode 36 1.500 to 2,000

Before the erase operation is started (i.e. during a normal viewingoperation), the conducting mesh 18 of the target may be +2 volts, forexample, and the initial charge on the storage grid 16 may be 0 volts.In carrying out the erasing operation, the potential of the erasing mesh18 is raised to +10 volts by means of an erase pulse, and the storagegrid 16 is raised to +8 volts. Under these conditions, the viewing beamelectrons charge the storage grid 16 until the voltage drops to 0.Removal of the positive pulse from the conducting mesh d8 drops thestorage grid 16 to +8 volts. This negative voltage on the storage grideffectively cuts off the viewing beam and thereby erases the chargepattern on the storage grid 16.

The erased pulse that is applied to the conducting mesh 18 effects areduction in the diameter of the viewing beam. Thus, the viewing beamhas a larger diameter in a viewing operation than in an erasingoperation. The area scanned by the writing beam has a still largerdiameter. As a consequence of this difference in the diameter of areasbombarded by the beams, two concentric annular portions of the storagegrid 16 are not erased during an erasing operation, but continue to bebuilt up in potential by the writing beam so that during a viewingcycle, one annular portion produces a bright ring on the viewing screen12 and the other annular portion gives rise to Voltage breakdown. Thisis objectionable in that the bright ring and the variable displayproduced by the breakdown may be considered to be a part of thedisplayed image, and in that their high degree of brightness may make itdiicult to view the desired image.

Prior attempts to solve this problem have not been entirely successful.One of such attempts has concerned the bright ring and has involved theapplication of a positive pulse to the target mesh 18. While this haswidened the erase pulse, it is characterized by several objectionsincluding the need for expensive external equipment that may fail inoperation.

This attempt, however, did not solve the voltage breakdown problem andthe resultant current transfer to the screen, or undesired screenexcitations through uncoated regions of the target. With respect to thevoltage breakdown, it is found that during the coating application forproducing the insulating coating comprising the storage grid 16, thecoating material unavoidably collects on the region 49 of the innersurface of tiange 50 (FIG. 2). This region is exposed to the writingbeam but beyond the range of the erasing beam. An attempted solution ofthe problem of voltage breakdown and resultant current transfer hasinvolved utilization of a masking technique during application of thetarget coating 16 to the target mesh 18. However, it is important thatall portions of the mesh 18 having apertures through which electrons maypass, be coated by the target material. To assure that the entireportion of target -rnesh 18 that is exposed to the several beams duringtube operation is coated, it is impossible from a practical standpoint,to avoid coating application to the area 49 of the flange 50. Whenattempts -are made to avoid coating the area 49, adjacent areas of thetarget mesh 18 sometimes are shielded by the mask and remain uncoated.

According to the invention, the objectionable bright ring on the screen12 and voltage breakdown, as well as objectionable effects from uncoatedportions of the target mesh, are eliminated in an economical anddependable manner. As shown in FIG. 2, the means for eliminating theobjectionable conditions referred to, may comprise a metal ring 52 fixedto an inturned flange portion 50 of the target support ring 54. Thesupport ring 54 may be insulatingly fixed (not shown) to the metalliccylinder 22 supporting the collector grid 24, in any suitable way, suchas by supporting means (not herein shown) described in co-pendingapplication Ser. No. 90,795, of R. G. Spangler, and assigned to theassignee of the present application. Cylinder 22 may be fixed to thecylindrical collimating electrode 34 in a similar manner.

The ring 52 is preferably made of a non-magnetic metal or alloy, such asstainless steel. However, any metal or alloy useful in electron tubestructures may be used, provided it can be welded easily or otherwisefixed to the support ange 50, and does not de-gas readily. Copper andaluminum cannot be welded easily and, therefore, are not preferred. Goldand platinum, in the absence of cost considerations, can also be used asthe composition of ring 52. The thickness of ring 52 is signicant inview of the relatively close spacing of about one-eighth inch betweenthe collector mesh 24 and the storage grid 16, and the relatively highvoltage difference (i.e., from 145 to 210 volts) between these elements.The aforementioned distance of one-eighth inch is reduced at the facingregions of the ange 50 and the mesh 24 by a value substantially equal tothe thickness of flange 50. This thickness is about mils in the interestof a rugged support for the target structure 1S, The ring S2 should beas thin as is consistent with ease of handling and spot welding and toavoid a further reduction in the space between the target support andthe collector mesh 24. We have found that ring 52 may have a thicknessof about 7 mils for good results.

The ring 52 should have a sufficiently large radial dimension, i.e., thedimension between its inner and Outer edges, to assure coverage of alltarget material deposited on the flange 50, during a coating operation.One way in which the insulating material of the storage grid 16 isapplied, is by evaporation. To assure that the entire area of the targetmesh 18, defined by the inner edge of flange S0 is coated, the evaporantis caused to overlap a portion ofthe flange 50. Since the exact degreeof overlap cannot be determined readily, it is preferred that the outerperiphery of ring 52 be closely adjacent to the cylindrical portion `ofthe support ring 54 with which flange 50 is integral and that the innerperiphery of ring 52 extend slightly over the electron bombarded portionof the target structure 15. In this way, there is assurance that allperipheral portions of the target that could contribute to the formationof a bright ring and voltage breakdown, are covered by the ring 52.While ring 52 is therefore in a position to receive electrons from oneor more electron beams, formed during tube operation, such electrons canbe dissipated readily through lead 56 which is connected to the rings 54and 52 (FIG. 1).

To provide further assurance that all areas of the target that mayremain bare of coating are covered by the ring 52, the ring ispreferably provided with a downwardly extending lip 58 as viewed in FIG.2. This not only assures coverage and desired shielding from the Writingbeam of any uncoated portion of the target mesh 18 adjacent to the innerperiphery of the ange 50, but also provides a round corner thatcontributes to a prevention of flash-over between ring 52, which is atthe potential of target mesh 18, and the collector mesh 24.

It should be noted that the purpose of ring 52 is not only to cover thecoated regions of the storage grid 16 that are incapable of satisfactoryerasure by prior art techniques, but also to cover any peripheralportions of the target mesh 18 adjacent to the inner edge of flange 50,that may not receive insulating coating materlal'of which the storagegrid 1o is composed, during a coating operation. If such uncoatedportions of the target mesh '18 are exposed to the electron beams,formed during tube operation, results harmful to tube operation arel1kely to occur. This is due to the fact that the target mesh 1S and thestorage grid 16 are impressed with appreciably different voltages duringtube operation. For example, the erasure pulse applied to the targetmesh 18 raises its potential to about +10 volts, while the storage grid16 (i.e., the charge holding coating) is raised only to about +8 volts.Removal of the erase pulse from the target mesh 18, reduces thepotential of this mesh to from .0 to +5 volts, while the potential ofthe storage grld 16 1s reduced to -8 volts. Since it is the -8 volts onthe storage grid 16 that accomplish cut off of the viewing beam, it isapparent that the viewing beam is not cut off at uncoated peripheralregions of the target mesh 18, which are at from 0 to +5 volts.

Failure to cut oft the erase beam at the uncoated peripheral region ofthe target mesh 1S during an erase operation, results in permeation ofelectrons from the erase beam through such regions. The electronspassing through such regions strike an annular portion of lthe phosphorscreen 12 causing the appearance of a bright ring thereon during theerase operation.

Any uncoated peripheral region of the target mesh 18 is alsoobjectionable during the writing and viewing operations. This is becauseany bare peripheral region is free of charge-holding insulation, and thewriting beam cannot impress an electrostatic charge pattern thereon. Asa matter of fact, the normal +2 volt potential on the target mesh 18,when coupled with the normal 0 volt potential on the storage grid 161,results in an attraction of electrons from the viewing beam to theuncoated portion of the target mesh 18, which attraction adverselyeffects the number of electrons passing through the coated region of thetarget and reaching the phosphor screen 12. During the viewingoperation, electrons of the viewing beam may pass through an uncoatedperipheral region of the target mesh 18 without modulation by theelectrostatic charge on the storage grid 16. Such unmodulated electronsmay be relatively numerous due to the fact that the potential on thestorage ,mesh 18 is +2 volts while that on the storage grid is 0i. Theresultant relatively large electron penetration of the uncoated regionsof the storage mesh 1S, causes the appearance of bright displays onportions of the phosphor screen 12 disposed in opposed relation to theuncoated regions of the storage mesh.

The ring 52 should preferably have an inner diameter that is no largerthan the diameter of the erasing beam. Preferably, the ring diametershould be smaller than the erasing beam diameter by an amount related tothe tolerance observed in mounting the erase (viewing) gun in the tube.One acceptable tolerance in this connection is about 0.1 inch for thetype of tube here involved. This tolerance value is indicative of thefact that the erase gun may be acceptably positioned so as to cause itsbea-m to extend in any lateral direction a distance of about 0.1 inchfrom an ideal location concentric with respect to the tube envelope. Thering 52, therefore, should preferably have an inner diameter that issmaller than the diameter of the erase beam, by a value of about 0.2inch. In a tube wherein the erase beam diameter is 4 inches, the innerdiameter of the ring 50 may be 3.8 inches.

It is apparent from the foregoing that the invention provides arelatively simple and reliable structure for avoiding undesiredexcitations of the phosphor screen of a display storage tube.

We claim:

1. A display storage tube comprising:

(a) a charge storage target structure, said structure including a targetsupport ring having an nturned annular support ange with an inner edge,and an apertured storage member having one face xed t one face of saidange,

(b) means for producing a writing beam of electrons directed towardssaid one face of said storage member,

(c) means for scanning said writing beam across said one face forcharging the same,

(d) a phosphor screen closely spaced from the opposite face of saidstorage member,

(e) means for directing a viewing beam towards said one face of saidstorage member, whereby electrons of said viewing beam selectively passthrough the apertures in said storage member and impinge upon saidscreen for producing a visible display thereon, and

(f) a conducting ring fixed to the other face of said flange and havinga lip extending over said edge, said ring extending over an annularportion of said apertured storage member adjacent to said edge, wherebysaid edge and said annular portion are shielded from the viewing beamand failure to coat said annular portion with insulating charge-holdingmaterial and failure to leave said edge uncoated are free from harmfuleffects upon the operation of said tube.

2. A display storage tube comprising,

(a) the charge storage target,

(1) said target comprising a metal mesh having acharge storage coatingon a portion of one side thereof, another portion of said side failingto receive said coating during a coating operation, and a target supportring having an annular inturned metal flange supporting the periphery ofsaid mesh and having an inner edge,

(b) a phosphor screen closely spaced from the opposite side of saidtarget,

(c) means for scanning a writing electron beam across said one side `ofsaid target including said another portion thereof that is free ofcoating material, said coating on the coated portion of said meshacquiring potential from said writing beam that is different from thepotential of said metal mesh free of coating,

(d) means for `directing to said one side of the target a viewing beamhaving a transverse area equal to the areas of said coated and uncoatedportions of the target, and

(e) imperforate conductive means covering said uncoated portion of thetarget, said means comprising a conducting 4ring fixed to said flange,said ring having a lip extending parallel to said edge and covering saiduncoated portion of said target,

(f) whereby said screen is preserved from electrons of said viewing beamthat have not been modulated by the charge storage coating of saidtarget.

References Cited by the Examiner UNITED STATES PATENTS JAMES W.LAWRENCE, Primary Examiner.

R. SEGAL, Examiner.

1. A DISPLAY STORAGE TUBE COMPRISING: (A) A CHARGE STORAGE TARGETSTRUCTURE, SAID STRUCTURE INCLUDING A TARGET SUPPORT RING HAVING ANINTURNED ANNULAR SUPPORT FLANGE WITH AN INNER EDGE, AND AN APERTUREDSTORAGE MEMBER HAVING ONE FACE FIXED TO ONE FACE OF SAID FLANGE, (B)MEANS FOR PRODUCING A WRITING BEAM OF ELECTRONS DIRECTED TOWARDS SAIDONE FACE OF SAID STORAGE MEMBER, (C) MEANS FOR SCANNING SAID WRITINGBEAM ACROSS SAID ONE FACE FOR CHARGING THE SAME, (D) A PHOSPHOR SCREENCLOSELY SPACED FROM THE OPPOSITE FACE OF SAID STORAGE MEMBER, (E) MEANSFOR DIRECTING A VIEWING BEAM TOWARDS SAID ONE FACE OF SAID STORAGEMEMBER, WHEREBY ELECTRONS OF SAID VIEWING BEAM SELECTIVELY PASS THROUGHTHE APERTURES IN SAID STORAGE MEMBER AND IMPINGE UPON SAID SCREEN FORPRODUCING A VISIBLE DISPLAY THEREON, AND (F) A CONDUCTING RING FIXED TOTHE OTHER FACE OF SAID FLANGE AND HAVING A LIP EXTENDING OVER SAID EDGE,SAID RING EXTENDING OVER AN ANNULAR PORTION OF SAID APERTURED STORAGEMEMBER ADJACENT TO SAID EDGE, WHEREBY SAID EDGE AND SAID ANNULAR PORTIONARE SHIELDED FROM THE VIEWING BEAM AND FAILURE TO COAT SAID ANNULARPORTION WITH INSULATING CHARGE-HOLDING MATERIAL AND FAILURE TO LEAVESAID EDGE UNCOATED ARE FREE FROM HARMFUL EFFECTS UPON THE OPERATION OFSAID TUBE.